The design of the LM UV-inkjet inks needs to aim for a combination of a high degree of curing, in order to have low residual amounts in the cured ink of ink compounds that can migrate, with the use of compounds that are low in viscosity but cannot migrate. The proprietary LM UV-inkjet ink formulations take all these restrictions into account and are even defined by them. This is illustrated in Figure 3.
LM UV-inkjet inks need the combination of low viscous but highly reactive monomers with diffusion-hindered photoinitiators. Highly reactive low viscous monomers are, for instance, monomers with two different polymerizable functions – of which the first is an acrylate and the second an ethylenically unsaturated polymerizable group, preferably selected from the group consisting of a vinyl ether group, an allyl ether group and an allyl ester group.
Diffusion-hindered photoinitiators and co-initiators are compounds that are designed to result in low migration levels, and – for inkjet – also are designed to have low viscosity. Different types of approaches are possible. Oligomeric and polymeric – as well as polymerizable initiators and co-initiators – have proven to be useful for formulating LM UV-inkjet inks. Each type has specific advantages towards the inkjet ink formulation and/or the lowest possible migration.
It is the combination of the above monomers and diffusion-hindered photoinitiators that results in a high degree of curing and a low amount of compounds that can migrate from the cured layer. The LM ink concept is summarized in Figure 4.